String is Nullable returns false

Question

Why does:

 string s = "";
 bool sCanBeNull = (s is Nullable);
 s = null;

sCanBeNull equate to false?

I'm writing a code generator and need to ensure every type passed to it is nullable, if it isn't already.

       //Get the underlying type:
       var type = field.FieldValueType;

        //Now make sure type is nullable:
        if (type.IsValueType) 
        {
            var nullableType = typeof (Nullable<>).MakeGenericType(type);
            return nullableType.FullName;
        }
        else
        {
            return type.FullName;
        }

Do I need to have to explicitly check for a string or am I missing something?

Answer 1

is tells you whether a value is of a particular type, or one derived from that particular type.

Nullable is a generic struct that allows for nullable versions of non-nullable values.

string is not a Nullable

To tell if a type can have null values use the fact that for all such types the default value is null, while for all other types it is not:

default(string) == null; // true